Characteristics of the catalytic activity of the enzymes are discussed in relation to their structure and the nature of their cofactors including metal ions. Metal ions exhibit various and important roles in the catalytic activity of the enzymes : in the formation of the enzyme-substrate complex, in the activation of the substrate molecule and in the activation of the enzyme catalytic site as a nucleophile or an acid-base catalyst.
One of the major functions of the essential metals in biological systems is their catalytic activities as metalloenzymes. In this section, discussions are mainly focused on the iron and copper enzymes. These transient metals are involved as cofactors in various enzymes or proteins, including oxidoreductases, oxygenases, electron carrier proteins, and oxygen carrier proteins. Much evidence suggest that there are three distinct states of copper in copper-containing enzymes, i. e., blue Cu (II), nonblue Cu (II), and diamagnetic Cu (II) -Cn (II) pair. The relation between the chemical state and biological function of the copper is discussed. Likewise, X-ray crystallographic analyses of the iron-containing electron carrier proteins, iron-sulfur proteins, reveal that there are three principal types of Fe-S clusters, namely, 1 Fe, 2 Fe-2S, and 4 Fe-4 S per center. Some of these clusters are also involved in various enzymes, such as xanthine oxidase, nitrogenase systems, hydrogenase, mitochondrial electron transport systems and so forth. In oxygenases too, iron and copper are the important cofactors ; most of dioxygenases as well as some of monooxygenases contain either iron or copper. The role of these transient metals in oxygenation reaction as a site of oxygen activation is also discussed.
The essential trace metals incorporated into metalloenzyme systems are Cr, Mn, Fe, Co, Cu, Zn, and Mo. Among other metal ions which are known to exist in biological systems in larger concentrations, Ca and Mg are also involved in enzymatic functions. The bonding forms in connecting substrate, enzyme, and metal were classified into three major categories. The stabilities of coordinate bonds were discussed in terms of the interactions between Lewis acids and bases of soft and hard nature. The typical coordination geometries of essential metals involved in biological functions were also given. The catalytic functions of nuclear metals were tentatively attributed to electronic, stereo-chemical, and oxidation-reduction effects and explained by referring to several well-qualified examples. The emphasis has been made on the hydrophobic field effect provided by apoenzymes for the development of catalytic functions of nuclear metal ions as well as on the cooperative catalytic effects by specific functional groups of apoenzymes placed in the active centers.
Behaviors of metal enzymes, especially of Fe-porphyrin ones are considered from the quantum-biological view point. In the introductory section, historical background of quantum biology and the inevitable role ought to be played by quantum biology in studying the enzyme reaction mechanism are described, in the light of the splendid success of quantum chemistry in explaining organic reaction mechanism. In Section 2, the physical and chemical properties of the porphin and Fe-porphyrins are explained on the basis of the electronic structures of these molecules, placing special emphasis on the HOMO and LUMO and also in relation to the interaction between the molecular orbitals of Fe and porphin. In Section 3, reaction mechanism of cytochrome P-450 is discussed. By comparing the transition energies and oscilaltor strengths calculated by means of the P-P-P method with the observed optical data, the stacked models are proposed as the plausible spacial orientation between P-450 and the substrates.
The significance of metal ions biological systems is currently obvious. During the past two decades, considerable progress in understanding the role of metals in the biological processes has rasulted from the studies of polymer-metal complexes on the ground of their structural analyses and catalytic behaviors. However, the author denies the idea that only a macromolecular complex can have metalloenzyme like properties it is an open question whether or not a macromolecule is required. It is the purpose of this review to answer the question. The author shall review the related studies historically in several branches, and emphasize the importance of thermodynamic properties in the polymer-metal complexes.
Recent investigations of nitrogenase and model systems are briefly reviewed. The protein components of the enzyme system nitrogenase have been characterised from several sources. Examination by physico-chemical methods is revealing an elaborate reaction mechanism involving electron transport from one protein to the other promoted by reaction with Mg-ATP. The evidence that molybdenum in nitrogenase may play a direct role for nitrogen fixation is shown in some chemical model systems.
It is of particular importance to understand the reaction of oxygenases not only in view of the biological significance but also in the synthetic chemical point of view. Many oxygenases contain metal ions, which participate in the reaction center to play an important rôle for activation process of the reactions. The molecular mechanisms for the reactions of oxygenases are still remained obscure, although an “oxenoid mechanism” is now generally believed for the reactions of monooxygenases. In spite of many attempts to illucidate the enzyme reactions through various metal complex-catalyzed oxygenation of organic molecules as models for the monooxygenases, nearly nothing has been made clear even on the mechanisms of the model systems, particularly for the aromatic hydroxylation. It is suggested that FeO is a possible reactive species in common with the model aromatic hydroxylation systems. In some models for the reactions of dioxygenases quite high selectivity has been recognized and peroxy complex intermediates are considered for these models, and some such a model peroxy interme-diates are isolated. Some characteristics, chemical properties of model intermediates, and plausible mechanisms for these reactions are discussed.
Principles for construction of model compounds of hemoproteins are described. Strategies of the synthetic hemes are discussed for the approaches ; synthetic hemes derived from proto-heme, heme of the polymer matrix and synthetic hemes newly designed.
During the last several years, attempts to furnish the environment (ligand) of metal complexes with chirality-recognizing function, one of the most important features of enzymes, have led to the development of highly enantioselective catalysts (up to 96%ee) which catalyze various reactions involving hydrogenation, hydrosilylation, hydroformylation, codimerization, cross coupling, cyclopropanation, and kinetic resolution. These attempts are reviewed on the type of the information carriers, and some aspects are discussed for designing excellent catalysts with a higher enantioselectivity and catalytic activity.